It didn't work for me, as my setting had been set that way from the get go. My ability to load my books just abruptly stopped. And when they did, the removed all my books from my iPhone. They're all in my Books library on my Mac just fine, but they no longer show up in the Finder Sync window Books tab.
So far all the posts on this subject I've come across have all be closed by Apple, but with no real solution offered. I spent almost 3 hours with Support last week, and nothing. He couldn't figure it out, nor did he offer a solution to those who wished to offload their Books library on a separate hard drive. So, I'm beginning to think this bug is known, but is being ignored. I have over 2500 epubs, pdfs, etc., and there is no way I will be able to put them in Apple's cloud, nor should you or anyone have too, I'd have to rob a bank to pay their monthlies. We have been able to sync between devices in the past, so why not now? And if this is an underhanded way to force us into an iCloud subscription, then this is the last iPhone I buy. Too much money to get jerked around like a brand loyal fool lap dog.
As an aside, I dealt with an Apple Music subscription for four years that put my music library through the shredder and left me with over 3000 music files with no tag info. No names, titles, etc.. You know how I fixed it? I canceled my subscription. And walah! Four years of headaches gone. My Music library returned to normal.
[Edited by Moderator]